Asymmetric Diels-Alder reactions catalyzed by chiral lanthanide(III) trifluoromethanesulfonates. Unique structure of the triflate and stereoselective synthesis of both enantiomers using a single chiral source and a choice of achiral ligands

Abstract

Chiral lanthanide trifluoromethanesulfonates (triflates) were developed and the unique structure of the triflates was revealed. In the presence of a catalytic amount of the triflate, acyl-1,3-oxazolidin-2-ones reacted with cyclopentadiene to afford the Diels-Alder adducts in high yields and with high enantiomeric excesses. According to the reactions, both enantiomers of the Diels-Alder adducts were stereoselectively prepared by using a single chiral source, R-(+)-binaphthol, and a choice of achiral ligands. When 3-acetyl-1,3-oxazolidin-2-one was combined with the original catalyst system consisting of lanthanide triflate, R-(+)-binaphthol, and cis-1,2,6-trimethylpiperidine, a new catalyst was generated (catalyst A). In the presence of this catalyst, 3-acyl-1,3-oxazolidin-2-ones reacted with cyclopentadiene to afford the endo adducts in high enantiomeric excesses. The absolute configuration of the products was 2S, 3R. On the other hand, when 3-phenylacetylacetone (PAA) was mixed with the original catalyst system (catalyst B), reverse enantiofacial selectivities were observed and endo adducts with the absolute configuration 2R, 3S were obtained in high enantiomeric excesses.